It is generally desirable that components used in automotive and aerospace applications be manufactured from as few parts as is compatible with the final use of those components. One method of manufacturing parts which meets this requirement is to form a single sheet of metal into a part using a die set. The complexity of shape of parts which can be formed in this way is, however, limited by the mechanical properties of the sheet metal which is formed in the die set. On the one hand, it may be too brittle; on the other, it may be too ductile. In either case, formability would be limited. Previously, the present inventors discovered that solution heat treating a sheet of metal and then rapidly forming it into a part in a cold die set improves the formability of the metal, allowing more complex-shaped components to be manufactured from a single sheet. Such components therefore no longer need to be formed as a multi-part assembly.
This process is disclosed in WO 2010/032002 A1, which discloses a method of forming aluminium alloy sheet components, using a solution heat treatment, cold die forming and quenching (HFQ (RTM)) process. The temperature of a sheet of metal alloy as it goes through such a process is shown in FIG. 1. Essentially, this existing HFQ (RTM) process involves the following steps:
(A) preheating a sheet metal workpiece to, or above, the solution heat treatment (SHT) temperature range of the metal;
(B) soaking the workpiece at the preheat temperature to enable the material to be fully solution heat treated;
(C) transferring the workpiece to a cold die set and forming quickly at the highest possible temperature and at a high forming speed;
(D) holding the formed part in the cold die set for rapid cooling (cold die quenching) to achieve a super saturated solid solution (SSSS) material microstructure, desirable for post-form strength; and
(E) artificial or natural ageing of the formed part to obtain an improved strength for heat treatable materials.
At stage C, the workpiece is formed at a temperature close to the SHT temperature to enable the high ductility of the material to be employed in the forming of the part. At this high temperature, the workpiece is very soft, ductile and easy to deform. While this method therefore has certain advantages over earlier methods, including enabling the forming of parts which are complex in shape (complex parts) with SSSS microstructures desirable for high post-form strength, it also has certain drawbacks. These will now be described.
The workpiece is weak when it is near its SHT temperature. During forming of complex parts, certain areas of the workpiece are constrained by the die, while the others are forced to flow over the die. The flow of material from the areas which are held still in the die to the areas which are being stamped is restricted. This can result in localized thinning and tearing of the workpiece. This is because the forming process benefits less from the effect of strain hardening, which is weaker at higher temperatures particularly in the case of aluminium alloys. Strain hardens the metal so that areas of the workpiece which have been deformed become harder and hence stronger. This increases the ability of these deformed areas to pull other material in the region and draw that material into the die. The drawn in metal is itself strained and thus is hardened. This straining and hardening throughout a sheet inhibits localised thinning and leads to more uniform deformation. The greater the strain hardening, the greater the tendency to uniform deformation. With only weak strain-hardening, deformation is localized in areas of high ductility and draw-in is restricted, and so the incidence of localized thinning and failure may therefore increase. This degrades formability. To increase formability and strength in this process, the workpiece is formed in the dies at a very high speed in order to compensate for the weaker strain hardening at high temperatures by maximizing the effect of strain rate hardening.
The requirement for a high temperature to increase ductility and a high forming speed to increase strain hardening and strain rate hardening can lead to the following problems:
(i) A large amount of heat is transferred to the die set from the workpiece. As the forming process requires that the dies remain at a low temperature to achieve the quenching rate required to obtain a SSSS microstructure, they have to be artificially cooled, on the surface or by internal coolant-carrying channels (or otherwise). Repeated thermal cycles can lead to quicker degradation and wear of the dies.
(ii) For the mass-production of HFQ formed parts, the requirement that the dies be cooled complicates design, operation and maintenance of the dies, and increases die set cost.
(iii) The holding pressure and time in the die are higher, as the formed part has to be held in between the dies until it is cooled to the desired temperature. This uses more energy than processes with lower forming times and pressures and reduces forming efficiency and thus productivity.
(iv) The high forming speed can cause significant impact loads when the dies are closed during forming. Repeated loading can lead to damage and wear of the dies. It can also necessitate the use of high durability die materials, which increases the die set cost.
(v) Specialized high speed hydraulic presses are required for the process to provide the die closing force. These hydraulic presses are expensive, which limits application of HFQ processes.
It would be desirable to address at least some of these problems with existing HFQ processes.